No abstract

Magnetorheological/electrorheological dampers are complex devices and involve a large set of important material and geometric variable with mutual interactions between them. As such, reliable predictions of the damping force level in these devices are difficult to achieve. However, meaningful results can be obtained with significantly less effort through nondimensional parameters involving all key variables. Therefore, the goal of this study was to propose a robust set of nondimensional parameters for the purpose of modeling of magnetorheological/electrorheological dampers (and other flow-mode devices) as well as the characterization of data from experiments with magnetorheological/electrorheological devices. The proposed scheme employs five parameters characterizing the contribution of flow inertia, viscosity, and yield stress, as well as shear thinning/thickening effects to the damping force output of magnetorheological/electrorheological valves. It is the result of analysis of several constitutive models of non-Newtonian fluid models (Bingham plastic model, biviscous model, biplastic Bingham model, and Herschel–Bulkley model). Specifically, the goal was to derive analytical (exact) formulae for pressure gradient of all examined models excluding the Herschel–Bulkley model. In the Herschel–Bulkley model, the nondimensional relationship between pressure gradient and flow rate is given in a power-law form, and the analytical (exact) solution cannot be obtained. Prior art included analytical (exact) solutions for the Bingham plastic model only. In the most generic form, the expressions can be useful for designing magnetorheological/electrorheological flow-mode devices. Exemplary calculations of the damping force output are presented in this article for a custom single-gap magnetorheological piston. The piston contains a semi-bypass feature in the annulus to allow for low-breakaway forces at near-zero piston velocity inputs. The steady-state calculations are presented for two exemplary damper units, and the model is validated against experimental data. Finally, the expressions allow one to easily characterize flow data into separate regimes of damper operation by means of the proposed scheme.

Hysteresis is one of key factors influencing the output of magnetorheological (MR) actuators. The actuators reveal two primary sources of hysteresis. The hydro(mechanical) hysteresis can be related to flow dynamics mechanisms and is frequency- or rate-dependent. For comparison, the magnetic hysteresis is an inherent property of ferromagnetic materials forming the magnetic circuit of the actuators. The need for a good quality hysteresis model has been early recognized in studies on MR actuators; however, few studies have provided models which could be used in the design stage. In the paper we reveal a hybrid multiphysics model of a flow-mode MR actuator which could be used for that purpose. The model relies on the information which can be extracted primarily from material datasheets and engineering drawings. We reveal key details of the model and then verify it against measured data. Finally, we employ it in a parameter sensitivity study to examine the influence of magnetic hysteresis and other relevant factors on the output of the actuator.

In this study, a mathematical model of a monotube magnetorheological (MR) shock absorber is presented and verified with an emphasis on leakage flow mechanisms and their impact on the damping force output. The model can be used in shock absorber design studies as well as vehicle simulations. To copy the force increase with yield stress, the authors employed the generic biplastic Bingham model for capturing the hydraulic resistance of the annular flow path in the piston. Moreover, the authors considered the impact of high-speed losses, fluid chamber compressibility, cavitation, elastic deformation of cylinder, fluid inertia, floating piston inertia, gas chamber pressure and Coulomb friction between damper components and the cylinder. The presented MR shock absorber model of is verified against experimental data involving three prototype shock absorber units. One shock absorber unit was a conventional unit with only one annular flow path, the second one employed the thru-core flow bypass for force roll-off at low piston velocities. The third unit utilized a so-called flux bypass to lower the magnetic field strength in the annulus to initiate the flow of MR fluids at lower yielding pressures across the piston. The flux bypass was located in the annulus. Except for the bypass features, all units were identical. All secondary flow features affect on the damping force at low piston velocities in particular. The experiment covered all key flow regimes of MR shock absorber operation from low speed to high speed. The results show that the proposed approach is capable of capturing key characteristics across the examined range of piston velocities and coil current levels.

In this paper the authors present results of a magnetorheological (MR) damper prototype development and performance evaluation study. The damper is a device functioning in the so-called squeeze-mode of MR fluid flow regime of operation. By principle, in a squeeze-mode damper the control (working) gap height varies according to the prescribed displacement or force input profile. Such hardware has been claimed to be well suited to small-amplitude vibration damping applications. However, it is still in its infancy. Its potential seems appealing yet unclear. Accordingly, the authors reveal performance figures of the damper complemented by numerical finite-element simulations of the electro-magnetic circuit of the device. The numerical results are presented in the form of maps of averaged magnetic flux density versus coil current and gap height as well as magnetic flux, inductance, and cogging force calculations, respectively. The simulated data are followed by experimental evaluation of the damper performance incorporating plots of force versus piston displacement (velocity) across a prescribed range of excitation inputs. Moreover, some insight into transient (unsteady) characteristics of the device is provided through testing results involving transient currents.

Abstract:The study deals with the pinch mode of magnetorheological (MR) fluids' operation and its application in MR valves. By applying the principle in MR valves a highly non-uniform magnetic field can be generated in flow channels in such a way to solidify the portion of the material that is the nearest to the flow channel's walls. This is in contrary to well-known MR flow mode valves. The authors investigate a basic pinch mode valve in several fundamental configurations, and then examine their magnetic circuits through magnetostatic finiteelement (FE) analysis. Flux density contour maps are revealed and basic performance figures calculated and analysed. The FE analysis results yield confidence in that the performance of MR pinch mode devices can be effectively controlled through electromagnetic means.

Controllable dampers based on smart fluids contain internal passages through which the working fluid flows and wherein the controlled pressure drop occurs under the influence of magnetic or electric fields. In this paper, the dynamics of such dampers are analysed through a series of theoretical dynamic models of increasing detail and complexity. The models capture the medium- and high-frequency dynamics of the damping force output of the damper and include the lumped mass of the fluid contained in the internal flow passages, the piston and rod assembly mass, and the compressibility of the fluid and pressurized gas contained within the chambers of the damper. The models are derived in state-space form from which transfer functions and natural frequencies are obtained analytically and then calculated for each of the systems. The results are presented in the form of frequency responses (Bode plots). Finally, the effects of the key geometric parameters of the damper and of the relevant fluid properties on the damper force output dynamics are presented and discussed.

The so-called squeeze flow involves a magnetorheological (MR) fluid sandwiched between two planar surfaces setting up a flow channel. The height of the channel varies according to a prescribed displacement or force profile. When exposed to a magnetic field of sufficient strength MR fluids develop a yield stress. In squeeze-mode devices the yield stress varies with both the magnetic field magnitude and the channel height. In this paper an unsteady flow model of an MR fluid in squeeze mode is proposed. The model is developed in Ansys Fluent R16. The MR material flow model is based on the apparent viscosity approach. In order to investigate the material's behaviour the authors prepared a model of an idealized squeeze-mode damper in which the fluid flow is enforced by varying the height of the channel. Using mesh animation, the model plate is excited, and as the mesh moves, the fluid is squeezed out of the gap. In the simulations the model is subjected to a range of displacement inputs of frequencies from 10 to 20 Hz, and local yield stress levels up to 30 kPa. The results are presented in the form of time histories of the normal force on the squeezing plate and loops of force vs. displacement (velocity).